Charcoal ashes should only be handled after they are completely cooled. Once cooled, they can be collected in a non-combustible container such as an old coffee can or an empty, dry paint can. Or you can wrap the ashes in aluminum foil — a perfect reuse for food-contaminated foil, which cannot be recycled. The contained ashes should then be placed in the trash for pick-up. If you were thinking about spreading the ashes outside, first consider where those ashes came from.

Ash can make excellent fertilizer when alkaline soil is needed, but most charcoal ash should not be spread in your garden or compost. Popular brands of charcoal briquettes, which represent 93 percent of all charcoal shipped, contain additives that can affect the ash’s value as fertilizer and may pose a human health risk. Briquettes begin with scrap wood, but coal, borax, lighter fluid, and fillers may be added before the whole thing is pressed into a pillow shape. The shape and additives ensure an effective, consistent product.

Lump charcoal, the alternative to briquettes, also begins with scrap wood, but chemicals aren’t added during the manufacturing process. It is essentially burned wood and is therefore safe to add to your compost. Lump charcoal represents just 7 percent of charcoal shipped, but its use has steadily increased for at least a decade. Consider choosing it next time you grill; afterward, you can dispose of the ashes in your garden or compost pile. Or, skip charcoal entirely and go with the greener grilling option: a gas grill.

I've read that grilled food causes carcinogenic elements in the part of the food that gets blackened, regardless of the method of heat-gas or charcoal. So I think it's something that should be done very sparingly or not at all. This article tells about that in an easy to understand way http://www.bonappetit.com/test-kitchen/cooking-tips/article/is-grilling-good-for-you-or-bad-here-s-what-science-says